Light emitting diodes with zinc oxide current spreading and light extraction layers deposited from low temperature aqueous solution

Abstract

A method for fabricating a Light Emitting Diode (LED) with increased light extraction efficiency, comprising providing a III-Nitride based LED structure comprising a light emitting active layer between a p-type layer and an n-type layer; growing a Zinc Oxide (ZnO) layer epitaxially on the p-type layer by submerging a surface of the p-type layer in a low temperature aqueous solution, wherein the ZnO layer is a transparent current spreading layer; and depositing a p-type contact on the ZnO layer. The increase in efficiency may be more than 93% with very little or no increase in cost.

What is claimed is: 
     
       1. A method of fabricating ZnO layers on a III-Nitride light emitting diode (LED), comprising:
 obtaining the III-Nitride LED comprising a light emitting active layer between a III-Nitride p-type layer and a III-Nitride n-type layer; 
 obtaining a first aqueous solution having a first composition containing dissolved Zn(II); 
 heating the first aqueous solution to a first temperature less than 100° C.; 
 synthesizing and depositing an epitaxial ZnO seed layer, comprising submerging a III-nitride surface of the III-Nitride LED in the first aqueous solution at the first temperature, changing a pH of the first aqueous solution after the submerging to form a first pH, and using a chemical reaction involving the dissolved Zn(II) under conditions including the first pH, the first composition, and the first temperature, to form the epitaxial ZnO seed layer on the III-Nitride surface of the III-Nitride LED; 
 annealing the epitaxial ZnO seed layer at a second temperature higher than 100 degrees Celsius; 
 after the annealing, synthesizing and depositing one or more ZnO layers, comprising submerging the epitaxial ZnO seed layer in a second aqueous solution having a second pH and a second composition containing dissolved Zn(II), to form the one or more ZnO layers on the epitaxial ZnO seed layer; and 
 fabricating an n-contact to the n-type III-nitride layer and fabricating a p-contact to the one or more ZnO layers. 
 
     
     
       2. The method of  claim 1 , wherein a maximum temperature of the aqueous solution is less than 100° C. 
     
     
       3. The method of  claim 1 , wherein the dissolved Zn(II) in the first and second aqueous solutions is supplied by dissolving a water soluble salt of Zn(II). 
     
     
       4. The method of  claim 1 , wherein the dissolved Zn(II) in the first and second aqueous solutions is supplied by dissolving ZnO. 
     
     
       5. The method of  claim 1 , wherein the first and second aqueous solutions are growth solutions and deposition of the one or more ZnO layers is performed in a plurality of steps, such that, in any one of the steps, the reaction of the dissolved Zn(II) to form ZnO is caused, intensified, or otherwise controlled by an increase in the temperature of the first and second aqueous solutions and/or by a change in the first and second pH of the first and second aqueous solutions. 
     
     
       6. The method of  claim 1 , further comprising controlling a morphology of the one or more ZnO layers by creating one or more roughened, patterned, and/or structured surfaces of the one or more ZnO layers suitable for enhancing light extraction of light emitted by the LED. 
     
     
       7. The method of  claim 6 , wherein the creation of the roughened, patterned, and/or structured surfaces is by one or more steps involving a removal of material from the preformed one or more ZnO layers by means of physical or chemical etching. 
     
     
       8. The method of  claim 6 , wherein the roughened, patterned, and/or structured surfaces of the one or more ZnO layers are created during the step of synthesizing the ZnO layer. 
     
     
       9. The method of  claim 7 , wherein conditions of the physical or chemical etching used result in the ZnO surfaces of the one or more ZnO layers comprising hexagonal pyramids and/or pits exposing semipolar planes of the one or more ZnO layers. 
     
     
       10. The method of  claim 6 , wherein the roughened, patterned, and/or structured surfaces are a result of conditions used during the step of synthesizing the one or more ZnO layers. 
     
     
       11. The method of  claim 10 , wherein the conditions used during the step of depositing or synthesizing the one or more ZnO layers result in the ZnO surfaces comprising hexagonal pyramids and/or pits exposing semipolar planes of the one or more ZnO layers. 
     
     
       12. The method of  claim 6 , further comprising:
 etching the III-Nitride surface, or creating a template layer on the III-Nitride surface; and 
 performing the step of synthesizing and depositing the epitaxial ZnO seed layer on the template layer or on the etched III-Nitride surface, wherein the roughened, patterned, and/or structured ZnO surfaces are a result of the etching or template creation step performed prior to the step of synthesizing the ZnO layers. 
 
     
     
       13. The method of  claim 1 , wherein a morphology of the one or more ZnO layers produced is modified by an addition of one or more additives to the aqueous solution, wherein the one or more additives are selected from metal citrate salts, citric acid, surfactants, polymers, biomolecules, and other molecules that interact with one or more surfaces of the one or more ZnO layers. 
     
     
       14. The method of  claim 13 , wherein the morphology of the one or more ZnO layers produced by the addition of the one or more additives to the aqueous solution enhances light extraction from the LED. 
     
     
       15. The method of  claim 1 , wherein the second aqueous solution is a growth solution, and one or more components are supplied in the second aqueous solution for a purpose of incorporating one or more dopants into the one or more ZnO layers. 
     
     
       16. The method of  claim 15 , wherein at least one of the dopants is selected from the group consisting of B, Al, Ga, and In. 
     
     
       17. The method of  claim 15 , wherein incorporation of the dopants modifies one or more electrical properties of the one or more ZnO layers. 
     
     
       18. The method of  claim 1 , wherein at least one annealing step is performed subsequent to the step of depositing the one or more ZnO layers in the aqueous solution. 
     
     
       19. The method of  claim 18 , wherein the annealing step modifies one or more electrical properties of the one or more ZnO layers. 
     
     
       20. The method of  claim 18 , wherein the annealing step comprises heating the one or more ZnO layers to a temperature greater than 200 degrees Celsius. 
     
     
       21. The method of  claim 1 , wherein the one or more ZnO layers have a sheet resistance of no more than 12.5 Ohms per square. 
     
     
       22. The method of  claim 1 , wherein the depositing of the one or more ZnO layers comprises depositing multiple ZnO layers, wherein each ZnO layer is annealed prior to deposition of the next ZnO layer. 
     
     
       23. The method of  claim 22 , wherein each of the ZnO layers is annealed at a different temperature. 
     
     
       24. The method of  claim 1 , wherein the one or more ZnO layers are deposited on a bulk Gallium Nitride (GaN) substrate. 
     
     
       25. The method of  claim 1 , wherein the one or more ZnO layers have at least a 4 micrometer thickness. 
     
     
       26. The method of  claim 1 , performed in a reactor comprising a first zone and a second zone, wherein:
 the Zn(II) is dissolved in the first zone, and 
 the ZnO layers are synthesized in the second zone, 
 a nucleation rate of the ZnO in the epitaxial ZnO layer and in the one or more ZnO layers is controlled by varying any combination of the first and second aqueous solutions including complexing ligands, a first temperature of the first zone and a second temperature of the second zone, a heating or a cooling rate, or the heating and the cooling rate, of the first zone and the second zone, and a rate at which the aqueous solution moves between the first zone and the second zone, and 
 the synthesis of the ZnO in the epitaxial ZnO layer and in the one or more ZnO layers is caused by a reduction in solubility of the ZnO in the first and second aqueous solutions at the second temperature of the second zone compared to the first temperature of the first zone as a result of the complexing ligands acting to increase the ZnO solubility at the first temperature more than at the second temperature. 
 
     
     
       27. A method of fabricating ZnO layers on a III-Nitride light emitting diode (LED), comprising:
 obtaining the III-Nitride LED comprising a light emitting active layer between a III-Nitride p-type layer and a III-Nitride n-type layer; 
 obtaining a first aqueous solution having a first composition containing dissolved Zn(II); 
 heating the first aqueous solution to a first temperature less than 100° C.; 
 synthesizing and depositing an epitaxial ZnO seed layer, comprising submerging a III-nitride surface of the III-Nitride LED in the first aqueous solution at the first temperature, adding ammonium hydroxide to the first aqueous solution after the submerging and forming a first pH of the first aqueous solution after the submerging, and using a chemical reaction involving the dissolved Zn(II) under conditions including the first pH of the first aqueous solution, to form the epitaxial ZnO seed layer on the III-Nitride surface of the III-Nitride LED; 
 annealing the epitaxial ZnO seed layer at a temperature higher than 100 degrees Celsius; 
 after the annealing, synthesizing and depositing one or more ZnO layers, comprising submerging the epitaxial ZnO seed layer in a second aqueous solution having a second pH and a second composition containing dissolved Zn(II), under conditions including the second pH to form the one or more ZnO layers on the epitaxial ZnO seed layer; 
 fabricating an n-contact to the n-type III-nitride layer and fabricating a p-contact to the one or more ZnO layers.